(1) Field of the Invention
This invention relates to conversion of analog signals to digital signals. In particular, it relates to a tracking analog-to-digital converter (ADC) providing analog-to-digital conversion.
(2) Description of the Prior Art
In the computer and information age, digital technology continues to increase in importance. With advances in technology, more and more everyday devices now incorporate digital technology and operate with digital signals. However, many of the real signals that are controlled, sampled and measured by digital systems exist not as digital signals but as analog signals, which must first be converted from analog to digital form before they can be incorporated into a digital system. Thus, an important device in the implementation of both digital and analog systems is the analog-to-digital converter (ADC or A/D converter), which is a device that converts an analog input signal into a digital number proportional to the input signal.
As many digital systems require an analog-to-digital conversion stage, for example digital audio or digital power supply applications, there is an ever-increasing need for improvement of such ADCs. Furthermore, as the clocking rate of such systems also continues to increase, there is a particular need for ADC devices with an improved ability to quickly and reliably convert analog signals into a digital form for such high-speed digital systems; i.e. there is a need to increase the clock rate and reduce response time of ADCs.
Additionally, in the interest of both production and operational costs, there is a further need for ADC device designs that allow for quick and efficient analog-to-digital conversion with high accuracy and high speed while operating under low power and without complex hardware. The design of a device that achieves all these competing goals in combination makes the design of such a device challenging.
In general, an ADC may be based on many different forms of analog measurement and may convert the analog signal based on different quantities such as voltages, currents or charges. Furthermore, there are many different types of A/D converters.
A tracking-ADC, which may also be referred to as a delta encoded ADC, is based upon a comparison of an analog input with its digital-to-analog (D/A) converted digital output. Generally, a tracking-ADC performs an incremental approximation in steps of one LSB (least significant bit). Thus, the time required by a tracking ADC to follow an input signal varies in relation to the magnitude of change of the respective input signal; thus, the name “tracking”, as the tracking ADC tracks the signal. The worst-case time for tracking its input occurs when the input signal changes by the maximum value, which then requires LSB stepping over the full input range (full scale). In such a case, the tracking-ADC requires 2N−1 steps to follow the input, where N is the total number of binary coded bits in the digital-converted output.
The tracking ADC offers the advantage of robustness against missing codes and monotony errors. Furthermore, the digital conversion of the tracking ADC can “follow” the input signal for small input signal changes on the order of an LSB with a single step, meaning that the tracking ADC has a fast response time for small input changes. However, the design characteristics of the associated comparator concerning speed and accuracy are most significant for the detection of such small input changes. More significantly, for large changes of the input signal, the tracking ADC requires several LSB steps to follow the input signal. This time needed to track a large change in the input signal is known as slewing time and can require up to a maximum of 2N−1 steps. More precisely, slewing can be considered to be the time that the ADC output requires to converge to its input via feedback.
As large changes in the input signal require a large number of conversion steps to track the input signal, there is a desire to reduce the conversion time required for each step in order to reduce the cumulative time. Generally, the limiting characteristic on which the possible reduction of the conversion step time of a tracking-ADC is dependent on is the time that the comparator needs to compare the input signal with the D/A-converted analog feedback signal. This time is known as the settling time of the comparator. Thus, if the ADC is clocked by a periodic clock signal, the minimum clock period is generally limited by the maximum settling time of the comparator. Typically, such ADCs are driven by a fixed clock and are thus called synchronous ADCs.
There is a need for an analog-to-digital converter that provides fast conversion times, has low energy consumption and has a low error rate so as not to require additional complex hardware for error correction.